Forced desorption of polymers from interfaces (original) (raw)
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Force spectroscopy of polymer desorption: theory and molecular dynamics simulations
Soft Matter, 2014
Forced detachment of a single polymer chain, strongly-adsorbed on a solid substrate, is investigated by two complementary methods: a coarse-grained analytical dynamical model, based on the Onsager stochastic equation, and Molecular Dynamics (MD) simulations with Langevin thermostat. The suggested approach makes it possible to go beyond the limitations of the conventional Bell-Evans model. We observe a series of characteristic force spikes when the pulling force is measured against the cantilever displacement during detachment at constant velocity vc (displacement control mode) and find that the average magnitude of this force increases as vc grows. The probability distributions of the pulling force and the end-monomer distance from the surface at the moment of final detachment are investigated for different adsorption energy ǫ and pulling velocity vc. Our extensive MD-simulations validate and support the main theoretical findings. Moreover, the simulation reveals a novel behavior: for a strong-friction and massive cantilever the force spikes pattern is smeared out at large vc. As a challenging task for experimental bio-polymers sequencing in future we suggest the fabrication of stiff, super-light, nanometer-sized AFM probe.
The Detachment of a Polymer Chain from a Weakly Adsorbing Surface Using an AFM Tip
Langmuir, 1999
Atomic force microscopy has been used to investigate the detachment of single polymer chains from surfaces and to measure the picoNewton forces required to extend the chain orthogonal to the surface. Such recent experiments show that the force-extension profiles provide interesting signatures which might be related to the progressive detachment of the chain from a surface. Using equilibrium scaling analysis, activation kinetics, and exactly solvable partition functions we predict force versus extension profiles for various extension rates. We also show how variation in the extension rate can distinguish heterogeneous monomer-surface contacts. The qualitative features that we predict, such as sawtooth force profiles with detachment forces which decrease with extension, maximal yielding forces at high extension rates, and featureless force profiles at large extension, are also seen in experiment.
Polymer desorption under pulling: A dichotomic phase transition
Physical Review E, 2009
We show that the structural properties and phase behavior of a self-avoiding polymer chain on adhesive substrate, subject to pulling at the chain end, can be obtained by means of a Grand Canonical Ensemble (GCE) approach. We derive analytical expressions for the mean length of the basic structural units of adsorbed polymer, such as loops and tails, in terms of the adhesive potential and detachment force, and determine values of the universal exponents which govern their probability distributions. Most notably, the hitherto controversial value of the critical adsorption exponent φ is found to depend essentially on the interaction between different loops. The chain detachment transition turns out to be of the first order, albeit dichotomic, i.e., no coexistence of different phase states exists. These novel theoretical predictions and the suggested phase diagram of the adsorption-desorption transformation under external pulling force are verified by means of extensive Monte Carlo simulations.
Force-induced desorption of a linear polymer chain adsorbed on an attractive surface
Europhysics Letters (epl), 2005
We consider a model of self-avoiding walk on a lattice with on-site repulsion and an attraction for every vertex of the walk visited on the surface to study force-induced desorption of a linear polymer chain adsorbed on an attractive surface and use the exact enumeration technique for analyzing how the critical force for desorption fc(T)f_c(T)fc(T) depends on the temperature. The curve fc(T)f_c(T)fc(T) gives the boundary separating the adsorbed phase from the desorbed phase. Our results show that in two dimensions where surface is a line the force fc(T)f_c(T)fc(T) increases monotonically as temperature is lowered and becomes almost constant at very low temperatures. In case of three-dimensions we, however, find re-entrance, i. e. fc(T)f_c(T)fc(T) goes through a maximum as temperature is lowered. The behaviour of the polymer chain at different values of temperature and force is examined by calculating the probability distribution of the height from the surface of the vertex at which external force is applied.
Macromolecules, 2009
We consider the properties of a self-avoiding polymer chain, adsorbed on a solid attractive substrate which is attached with one end to a pulling force. The conformational properties of such chain and its phase behavior are treated within a Grand Canonical Ensemble (GCE) approach. We derive theoretical expressions for the mean size of loops, trains, and tails of an adsorbed chain under pulling as well as values for the universal exponents which describe their probability distribution functions. A central result of the theoretical analysis is the derivation of an expression for the crossover exponent φ, characterizing polymer adsorption at criticality, φ = α − 1, which relates the precise value of φ to the exponent α, describing polymer loop statistics. We demonstrate that 1 − γ11 < α < 1 + ν, depending on the possibility of a single loop to interact with neighboring loops in the adsorbed polymer. The universal surface loop exponent γ11 ≈ −0.39 and the Flory exponent ν ≈ 0.59. We present the adsorption-desorption phase diagram of a polymer chain under pulling and demonstrate that the relevant phase transformation becomes first order whereas in the absence of external force it is known to be a continuous one. The nature of this transformation turns to be dichotomic, i.e., coexistence of different phase states is not possible. These novel theoretical predictions are verified by means of extensive Monte Carlo simulations.
Detachment of semiflexible polymer chains from a substrate: a molecular dynamics investigation
The Journal of chemical physics, 2014
Using Molecular Dynamics simulations, we study the force-induced detachment of a coarse-grained model polymer chain from an adhesive substrate. One of the chain ends is thereby pulled at constant speed off the attractive substrate and the resulting saw-tooth profile of the measured mean force ⟨f⟩ vs height D of the end-segment over the plane is analyzed for a broad variety of parameters. It is shown that the observed characteristic oscillations in the ⟨f⟩-D profile depend on the bending and not on the torsional stiffness of the detached chains. Allowing for the presence of hydrodynamic interactions (HI) in a setup with explicit solvent and dissipative particle dynamics-thermostat, rather than the case of Langevin thermostat, one finds that HI have little effect on the ⟨f⟩-D profile. Also the change of substrate affinity with respect to the solvent from solvophilic to solvophobic is found to play negligible role in the desorption process. In contrast, a changing ratio ε(s)(B)/ε(s)(A)...
Pulling an adsorbed polymer chain off a solid surface
The European Physical Journal E, 2009
The thermally assisted detachment of a self-avoiding polymer chain from an adhesive surface by an external force applied to one of the chain ends is investigated. We perform our study in the "fixed height" statistical ensemble where one measures the fluctuating force, exerted by the chain on the last monomer when a chain end is kept fixed at height h over the solid plane at different adsorption strength ǫ. The phase diagram in the h − ǫ plane is derived both analytically and by Monte Carlo simulations. We demonstrate that in the vicinity of the polymer desorption transition a number of properties like fluctuations and probability distribution of various quantities behave differently, if h rather than f is used as an independent control parameter. PACS numbers: 82.35.Gh Polymers on surface; adhesion -64.60.A -Specific approaches applied to studies of phase transitions -62.25.+g Mechanical properties of nanoscale systems
Forcing adsorption of a tethered polymer by pulling
Journal of Statistical Mechanics: Theory …, 2010
We present an analysis of a partially directed walk model of a polymer which at one end is tethered to a sticky surface and at the other end is subjected to a pulling force at fixed angle away from the point of tethering. Using the kernel method, we derive the full generating function for this model in two and three dimensions and obtain the respective phase diagrams.
Force-induced desorption and unzipping of semiflexible polymers
Phys. Rev. Lett. , 2006
The thermally assisted force-induced desorption of semiflexible polymers from an adhesive surface or the unzipping of two bound semiflexible polymers by a localized force are investigated. The phase diagram in the force-temperature plane is calculated both analytically and by Monte Carlo simulations. Force-induced desorption and unzipping of semiflexible polymers are first order phase transitions. A characteristic energy barrier for desorption is predicted, which scales with the square root of the polymer bending rigidity and governs the initial separation process before a plateau of constant separation force is reached. This leads to activated desorption and unzipping kinetics accessible in single molecule experiments.
Non-equilibrium dynamics of single polymer adsorption to solid surfaces
Journal of Physics: Condensed Matter, 2009
Adsorption of polymers to surfaces is crucial for understanding many fundamental processes in nature. Recent experimental studies indicate that the adsorption dynamics is dominated by non-equilibrium effects. We investigate the adsorption of a single polymer of length N to a planar solid surface in the absence of hydrodynamic interactions. We find that for weak adsorption energies the adsorption time scales ∼ N (1+2ν)/(1+ν) , where ν is the Flory exponent for the polymer. We argue that in this regime the single chain adsorption is closely related to a field-driven polymer translocation through narrow pores. Surprisingly, for high adsorption energies the adsorption time becomes longer, as it scales ∼ N (1+ν) , which is explained by strong stretching of the unadsorbed part of the polymer close to the adsorbing surface. These two dynamic regimes are separated by an energy scale that is characterised by nonequilibrium contributions during the adsorption process.